Permeability acts as a core parameter governing the efficient and cost-effective development of deep coalbed methane (CBM) reservoirs. The evolution of permeability in deep CBM formations is predominantly driven by the coupled deformation of pore and fracture systems under in-situ stress, yet the intrinsic mechanisms behind this process have not been fully elucidated. In this work, permeability tests were carried out on cataclastic coal specimens in three orientations under both loading and unloading conditions with confining pressures. Experimental results reveal that coal permeability decreases exponentially with increasing effective stress (R2 is about 0.99; reduction is about 86%), exhibiting strong anisotropy and displays significant hysteresis during unloading. To interpret these phenomena, we establish a dual-pore fractal series model that uniquely integrates serial flow coupling between matrix pores and fractures and quantifies stress-driven changes in fractal dimension, tortuosity, and maximum pore size. The model successfully reproduces experimental results (mean relative error ≤ 4.2%) and provides mechanistic insights into stress-induced permeability evolution. Stress increases fractal dimension and tortuosity while reducing maximum pore size, rendering pore structures more complex and less conductive. Incomplete recovery of fractal parameters during unloading explains the observed hysteresis. This mechanistic framework, combining the experiment and theory, offers quantitative support for optimizing CBM extraction strategies.
Wu et al. (Tue,) studied this question.